An isolated switching power converter provides regulated power to an electronic device while providing galvanic isolation between the electronic device and an AC power source. Typically an isolated switching power converter comprises a transformer which comprises a primary winding coupled with the AC power source and a secondary winding coupled with an output of the converter circuit. The transformer provides galvanic isolation, and components which are coupled with the primary winding are collectively referred to as the primary side of the power converter circuit, while components which are coupled with the secondary winding are collectively referred to as the secondary side of the power converter circuit. The output provides a regulated voltage for an output load, which may comprise an electronic device.
A power stage switch is provided at the primary side which controls the delivery of energy to the output load. In the switch's first (closed) state, the primary winding is connected with the input voltage source and current is generated in the primary winding. Energy is stored in the transformer and an output capacitor or equivalent storage element in the secondary side delivers energy to the load. In the switch's second (open) state, no current flows in the primary winding. The energy stored by the transformer charges the storage element in the secondary side and supplies the load.
The primary side comprises a controller for operating the power stage switch. To achieve effective control it is necessary to provide a signal related to the output voltage to the primary side. In secondary side regulation, an optocoupler is provided which feeds back the secondary side output voltage to the primary side. However, an optocoupler takes up space, increases the cost of the circuit and can be unreliable. Another issue with the optocoupler based communication is the relatively low speed. Even though high speed safety isolated transmitter-receiver pairs are available on the market; they are prohibitively expensive for low cost consumer devices such as wall charger adapters.
In primary side regulation, an auxiliary winding is provided at the primary side and the voltage across the auxiliary winding reflects the voltage across the secondary winding. An optocoupler is therefore not required, thus making primary side regulation attractive as it avoids the extra expense, cost and reliability issues.
However, it is often desirable to communicate data from the secondary side to the primary side. An example where this is advantageous is the field of chargers which provide high speed charging. Rapid charging rates require careful monitoring of various parameters such as battery terminal voltage and battery temperature to prevent overcharging and damage to the battery cells, due to the relatively higher currents involved.
Data communication is also useful in other power conversion systems that need to quickly adjust the output regulation points for other reasons than for rapid charging. Examples include wireless charging, or generally any technology where regulation feedback from the secondary side to the primary side is required.
A communication link could also be used to provide other data to the primary side controller, such as determining which specific electronic device is connected to the power supply, determining the operational characteristics of a connected electronic device including, for example, the operating voltage level, current level, and/or operating mode (for example, shut-down mode, sleep mode, hibernation mode). The switching power converter may then adapt its switching operation to achieve different regulated output voltage and/or current to accommodate the detected electronic device and/or its operating mode. With such communication ability, the switching power converter can provide additional functionality, including the ability to accommodate a wide variety of different electronic devices.
Secondary side regulation already provides means for such communication via the optocoupler. However standard primary side regulation does not provide any way for such communication to take place. It is desirable to avoid the use of an optocoupler if possible because it represents a large overhead in terms of die area and cost.
It has been suggested, in U.S. Pat. No. 8,854,842, to provide a switch and a controller at the secondary side. The switch is operated in a dedicated messaging mode and during a dead time period. It switches on and off to cause voltage fluctuations across the secondary winding that are transferred to the primary side. The switching pattern is chosen to encode data to be input to the controller at the primary side.
The scheme of U.S. Pat. No. 8,854,842 relies on the ringing periods of the sensed voltages having subsided. This reduces the time during which data can be transmitted. However it is not practical to wait until the ringing subsides in any of the load conditions except for a no load mode. Whenever there is a significant load, primary side valley mode switching turns on the next switch cycle well before the ringing is subsided. Without doing this, it is not practical to deliver the power. The encoding of data will also be subject to error if ringing is persisting at the time when the switching is taking place.